Quantitative evaluation of vacuum conditions for void removal in flip-chip underfill encapsulation

Abstract

Void formation during underfill encapsulation remains a critical reliability challenge in flip-chip semiconductor packaging. This study quantitatively evaluates the effectiveness of vacuum-assisted degassing in eliminating entrapped voids within the underfill layer. The void-removal process was directly visualized during vacuum treatment, and image-based quantitative analysis was employed to assess degassing performance under wellcontrolled vacuum conditions. Experiments were conducted at three vacuum pressures (-0.03, -0.06, and -0.09 MPa) and holding times (1, 3, and 5 min), using the void-area fraction as the key performance metric. The results indicate that vacuum pressure is the primary factor governing void removal. A size-dependent behavior was observed, where larger voids responded more effectively to higher pressure differentials. To examine viscosity-related effects, three commercial underfill materials with distinct rheological properties were tested under identical vacuum conditions. Despite differences in viscosity, all materials achieved comparable final voidarea fractions after degassing, suggesting that, under sufficiently strong vacuum and adequate holding time, material viscosity exerts a minimal influence on the overall void-removal efficiency. These findings provide quantitative insight for optimizing underfill processes by prioritizing vacuum conditions, thereby enhancing packaging reliability through improved void control.